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Hadoop >> mail # user >> Re: Mutiple dfs.data.dir vs RAID0


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Re: Mutiple dfs.data.dir vs RAID0
Typical best practice is to have a separate file system per spindle.  If
you have a RAID only controller (many are), then you just create one RAID
per spindle.  The effect is the same.

MapR is unusual able to stripe writes over multiple drives organized into a
storage pool, but you will not normally be able to achieve that same level
of performance with ordinary Hadoop by using LVM over JBOD or controller
level RAID.  The problem is that the Java layer doesn't understand that the
storage is striped and the controller doesn't understand what Hadoop is
doing.  MapR schedules all of the writes to individual spindles via a very
fast state machine embedded in the file system.

The comment about striping increasing the impact of a single disk drive is
exactly correct and it makes modeling the failure modes of the system
considerably more complex.  The net result of the modeling that I and
others have done is that moderate to large RAID groups in storage pools for
moderate sized clusters (< 2000 nodes or so) is just fine.  For large
clusters of up to 10,000 nodes, you should probably limit RAID groups to 4
drives or less.

On Sun, Feb 10, 2013 at 7:39 PM, Marcos Ortiz <[EMAIL PROTECTED]> wrote:

>  We have seen in several of our Hadoop clusters that LVM degrades
> performance of our M/R jobs, and I remembered a message where
> Ted Dunning was explaining something about this, and since
> that time, we don't use LVM for Hadoop data directories.
>
> About RAID volumes, the best performance that we have achieved
> is using RAID 10 for our Hadoop data directories.
>
>
>
> On 02/10/2013 09:24 PM, Michael Katzenellenbogen wrote:
>
> Are you able to create multiple RAID0 volumes? Perhaps you can expose
> each disk as its own RAID0 volume...
>
> Not sure why or where LVM comes into the picture here ... LVM is on
> the software layer and (hopefully) the RAID/JBOD stuff is at the
> hardware layer (and in the case of HDFS, LVM will only add unneeded
> overhead).
>
> -Michael
>
> On Feb 10, 2013, at 9:19 PM, Jean-Marc Spaggiari<[EMAIL PROTECTED]> <[EMAIL PROTECTED]> wrote:
>
>
>  The issue is that my MB is not doing JBOD :( I have RAID only
> possible, and I'm fighting for the last 48h and still not able to make
> it work... That's why I'm thinking about using dfs.data.dir instead.
>
> I have 1 drive per node so far and need to move to 2 to reduce WIO.
>
> What will be better with JBOD against dfs.data.dir? I have done some
> tests JBOD vs LVM and did not find any pros for JBOD so far.
>
> JM
>
> 2013/2/10, Michael Katzenellenbogen <[EMAIL PROTECTED]> <[EMAIL PROTECTED]>:
>
>  One thought comes to mind: disk failure. In the event a disk goes bad,
> then with RAID0, you just lost your entire array. With JBOD, you lost
> one disk.
>
> -Michael
>
> On Feb 10, 2013, at 8:58 PM, Jean-Marc Spaggiari<[EMAIL PROTECTED]> <[EMAIL PROTECTED]> wrote:
>
>
>  Hi,
>
> I have a quick question regarding RAID0 performances vs multiple
> dfs.data.dir entries.
>
> Let's say I have 2 x 2TB drives.
>
> I can configure them as 2 separate drives mounted on 2 folders and
> assignes to hadoop using dfs.data.dir. Or I can mount the 2 drives
> with RAID0 and assigned them as a single folder to dfs.data.dir.
>
> With RAID0, the reads and writes are going to be spread over the 2
> disks. This is significantly increasing the speed. But if I put 2
> entries in dfs.data.dir, hadoop is going to spread over those 2
> directories too, and at the end, ths results should the same, no?
>
> Any experience/advice/results to share?
>
> Thanks,
>
> JM
>
>
> --
> Marcos Ortiz Valmaseda,
> Product Manager && Data Scientist at UCI
> Blog: http://marcosluis2186.posterous.com
> Twitter: @marcosluis2186 <http://twitter.com/marcosluis2186>
>